Magnetic storage device with humidity control device incorporating a differentially permeable membrane

ABSTRACT

Approaches for desiccant device within an enclosure protecting sensitive electronic equipment. Electronic equipment, such as a hard-disk drive (HDD), may comprise a desiccant device that possesses a water vapor permeability rate that matches or is substantially similar to the water vapor permeability rate of the enclosure. In this way, desiccant devices may prevent the desiccant material enclosed within from releasing large amounts of humidity into the interior of the electronic equipment. The desiccant device may be constructed using a laminate material that comprises an interior facing material and an exterior facing material. The exterior facing material permits water to diffuse faster therethrough than the interior facing material.

RELATED APPLICATION DATA

This application is related to U.S. patent application Ser. No. 13/212,038, entitled “Magnetic Storage Device with Dynamic Humidity Control System to Mitigate Water Vapor Transients,” invented by Charles Allan Brown et al., hereafter referred to as “the Brown patent application,” filed Aug. 17, 2011, the disclosure of which is incorporated by reference in its entirety for all purposes as if fully set forth herein.

This application is related to U.S. patent application Ser. No. 13/211,748, entitled “Magnetic Storage Device with Multi-Functional Component for Controlling Chemical and Water Vapor Therein,” invented by Charles Allan Brown, filed Aug. 17, 2011, the disclosure of which is incorporated by reference in its entirety for all purposes as if fully set forth herein.

FIELD OF THE INVENTION

Embodiments of the invention generally relate to a desiccant device, for use in a sensitive electronic device, which has a differentially permeable membrane.

BACKGROUND OF THE INVENTION

A hard-disk drive (HDD) is a non-volatile storage device that is housed in a protective enclosure and stores digitally encoded data on one or more circular disks having magnetic surfaces (a disk may also be referred to as a platter). When an HDD is in operation, each magnetic-recording disk is rapidly rotated by a spindle system. Data is read from and written to a magnetic-recording disk using a read/write head which is positioned over a specific location of a disk by an actuator.

A read/write head uses a magnetic field to read data from and write data to the surface of a magnetic-recording disk. As a magnetic dipole field decreases rapidly with distance from a magnetic pole, the distance between a read/write head and the surface of a magnetic-recording disk must be tightly controlled. An actuator relies on suspension's force on the read/write head to provide the proper distance between the read/write head and the surface of the magnetic-recording disk while the magnetic-recording disk rotates. A read/write head therefore is said to “fly” over the surface of the magnetic-recording disk. When the magnetic-recording disk stops spinning, a read/write head must either “land” or be pulled away onto a mechanical landing ramp from the disk surface. Because the read/write head flies so close to the surface of the magnetic-recording disk, the components within the HDD may be sensitive to moisture, contaminants, and other impurities and imperfections within the protective enclosure of the HDD.

Over time, the design of new HDD models has enabled lower and lower fly heights. As fly heights decrease, magnetic storage devices (such as HDDs) are at increased risk to experiencing harm from high internal water vapor concentration. High concentrations of water vapor can alter fly heights and thus impact error rates or even wear upon the magnetic-recording head. In addition, high concentration of water vapor can promote various undesirable processes such as corrosion. To illustrate, at 30° C. there is a relatively small change in the fly height if there is an increase in internal humidity; however, at 60° C. (which is a common internal operating temperate of a HDD in a data center), the effect is much more pronounced and typical air bearings of the magnetic read/write head will cause a significant decrease in the fly height as the humidity rises.

Desiccants may be used within electronic equipment (such as a HDD) to reduce concentrations of water vapor therein.

SUMMARY OF THE INVENTION

It is observed that desiccants rapidly equilibrate with their environment. In effect, desiccant buffer the humidity of their environment by absorbing or releasing water as needed to maintain equilibrium. The capacity of a typical desiccant used within a HDD far exceeds the amount of water the desiccant needs to absorb, or release, to maintain the interior of the HDD at a specific humidity. This property poses a problem in that if the desiccant has absorbed moisture during the storage of either the HDD itself or a system in which the HDD is installed, then the desiccant will act to maintain the humidity after the HDD is placed into operation. For a HDD at any humidity, the water vapor concentration will increase superlinearly with temperature. Although it is often assumed that when the temperature rises, the relative humidity of an enclosure will drop correspondingly, this is in fact true only if there are no materials in the enclosure that can hold dissolved or absorbed water. An all metal enclosure that is truly empty will exhibit a decrease in relative humidity with an increase in temperature; but if there are absorbers of water in the enclosure, then the change in relative humidity will be less, or even not at all.

If a HDD is stored in a humid environment, such as may be encountered during sea shipment or holding in a warehouse that is not air conditioned, the interior of the HDD will equilibrate with the ambient environment in about a month. If a dry desiccant is placed inside a HDD at the time of manufacture, the humidity rise in the desiccant to equal the ambient environment will take several months. However, once loaded with water, the desiccant will then cause the humidity to remain high when the HDD is placed into service. This is exacerbated by the use of diffusion restricting channels on breather filters to control ingress of water vapor, organic vapors, and corrodants into the HDD.

A highly permeable gasket will allow water vapor to dissipate from inside the HDD once it is in service. However, the same highly permeable gasket will also allow water to enter the HDD during storage or during shutdown. In principle, one can use a suitably actuated valve to bypass the breather diffusion restricting channel; however, such an actuated valve would add cost and complexity to the HDD. Further, flat “flapper” type valves are notoriously prone to sticking and would require high opening force after some time closed.

Advantageously, embodiments of the invention provide for a laminate that would permit water to diffuse at different rates depending on which side of the laminate is proximate to the higher concentration of water. Laminates of embodiments may be used to construct a desiccant device for enclosing a desiccant material. The desiccant device of an embodiment permits water to diffuse into the desiccant device at a faster rate than water is permitted to diffuse out of the desiccant device. In certain embodiments, the rate of which water vapor may diffuse out of the desiccant device is equal to or approximates the rate at which water vapor may diffuse out of the electronic equipment in which the desiccant device is disposed, thereby ensuring that the amount of water vapor within the interior of the electronic equipment does not exceed a certain threshold.

Embodiments discussed in the Summary of the Invention section are not meant to suggest, describe, or teach all the embodiments discussed herein. Thus, embodiments of the invention may contain additional or different features than those discussed in this section.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:

FIG. 1 is a plan view of an HDD according to an embodiment of the invention;

FIG. 2 is a plan view of a head-arm-assembly (HAA) according to an embodiment of the invention;

FIG. 3 is an illustration depicting several exemplary desiccant devices according to embodiments of the invention;

FIG. 4 is an illustration of a desiccant device having a diffusion channel according to an embodiment of the invention;

FIG. 5A is a graph that illustrates the interaction of relative humidity, absolute humidity ([H₂O]), and temperature;

FIG. 5B is a graph that illustrates a change in the fly height measured as a function of temperature and relative humidity inside of a HDD;

FIG. 6 is a graph that illustrates a change in the fly height measured as a function of temperature and relative humidity inside of a HDD;

FIG. 7A depicts a laminate from which at least a portion of a desiccant device according to an embodiment of the invention may be constructed;

FIG. 7B is an illustration of a desiccant device according to an embodiment of the invention; and

FIGS. 8A-8B are graphs illustrating data concerning water vapor diffusion through a laminate according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Approaches for a desiccant device for use within electronic equipment are described. The desiccant device permits water to diffuse into the desiccant device at a faster diffusion rate than water is permitted to diffuse out of the desiccant device. Embodiments may be used to prevent the desiccant material enclosed within the desiccant device from releasing large amounts of humidity into the interior of the electronic equipment. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the invention described herein. It will be apparent, however, that the embodiments of the invention described herein may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the embodiments of the invention described herein.

Physical Description of Illustrative Embodiments of the Invention

Embodiments of the invention may be used to prevent a desiccant, used within sensitive electronic equipment such as a hard-disk drive (HDD), from releasing large amounts of humidity into the interior of the sensitive electronic equipment. For purposes of providing a concrete example, particular embodiments of the invention shall be described with reference to a hard-disk drive (HDD). However, desiccants according to embodiments of the invention may be used within the interior of many types of electronic equipment. Consequently, embodiments of the invention need not be limited to use within a HDD, since embodiments of the invention have broader applicability.

In accordance with an embodiment of the invention, a plan view of a HDD 100 is shown in FIG. 1. FIG. 1 illustrates the functional arrangement of components of the HDD including a slider 110 b that includes a magnetic-recording head 110 a. The HDD 100 includes at least one head gimbal assembly (HGA) 110 including the head 110 a, a lead suspension 110 c attached to the head 110 a, and a load beam 110 d attached to the slider 110 b, which includes the head 110 a at a distal end of the slider 110 b; the slider 110 b is attached at the distal end of the load beam 110 d to a gimbal portion of the load beam 110 d. The HDD 100 also includes at least one magnetic-recording disk 120 rotatably mounted on a spindle 124 and a drive motor (not shown) attached to the spindle 124 for rotating the disk 120. The head 110 a includes a write element and a read element for respectively writing and reading information stored on the disk 120 of the HDD 100. The disk 120 or a plurality (not shown) of disks may be affixed to the spindle 124 with a disk clamp 128. The HDD 100 further includes an arm 132 attached to the HGA 110, a carriage 134, a voice-coil motor (VCM) that includes an armature 136 including a voice coil 140 attached to the carriage 134; and a stator 144 including a voice-coil magnet (not shown); the armature 136 of the VCM is attached to the carriage 134 and is configured to move the arm 132 and the HGA 110 to access portions of the disk 120 being mounted on a pivot-shaft 148 with an interposed pivot-bearing assembly 152.

With further reference to FIG. 1, in accordance with an embodiment of the present invention, electrical signals, for example, current to the voice coil 140 of the VCM, write signal to and read signal from the PMR head 110 a, are provided by a flexible cable 156. Interconnection between the flexible cable 156 and the head 110 a may be provided by an arm-electronics (AE) module 160, which may have an on-board pre-amplifier for the read signal, as well as other read-channel and write-channel electronic components. The flexible cable 156 is coupled to an electrical-connector block 164, which provides electrical communication through electrical feedthroughs (not shown) provided by an HDD housing 168. The HDD housing 168, also referred to as a casting, depending upon whether the HDD housing is cast, in conjunction with an HDD cover (not shown) provides a sealed, protective enclosure for the information storage components of the HDD 100.

With further reference to FIG. 1, in accordance with an embodiment of the present invention, other electronic components (not shown), including a disk controller and servo electronics including a digital-signal processor (DSP), provide electrical signals to the drive motor, the voice coil 140 of the VCM and the head 110 a of the HGA 110. The electrical signal provided to the drive motor enables the drive motor to spin providing a torque to the spindle 124 which is in turn transmitted to the disk 120 that is affixed to the spindle 124 by the disk clamp 128; as a result, the disk 120 spins in a direction 172. The spinning disk 120 creates a cushion of air that acts as an air-bearing on which the air-bearing surface (ABS) of the slider 110 b rides so that the slider 110 b flies above the surface of the disk 120 without making contact with a thin magnetic-recording medium of the disk 120 in which information is recorded. The electrical signal provided to the voice coil 140 of the VCM enables the head 110 a of the HGA 110 to access a track 176 on which information is recorded. Thus, the armature 136 of the VCM swings through an arc 180 which enables the HGA 110 attached to the armature 136 by the arm 132 to access various tracks on the disk 120. Information is stored on the disk 120 in a plurality of concentric tracks (not shown) arranged in sectors on the disk 120, for example, sector 184. Correspondingly, each track is composed of a plurality of sectored track portions, for example, sectored track portion 188. Each sectored track portion 188 is composed of recorded data and a header containing a servo-burst-signal pattern, for example, an ABCD-servo-burst-signal pattern, information that identifies the track 176, and error correction code information. In accessing the track 176, the read element of the head 110 a of the HGA 110 reads the servo-burst-signal pattern which provides a position-error-signal (PES) to the servo electronics, which controls the electrical signal provided to the voice coil 140 of the VCM, enabling the head 110 a to follow the track 176. Upon finding the track 176 and identifying a particular sectored track portion 188, the head 110 a either reads data from the track 176 or writes data to the track 176 depending on instructions received by the disk controller from an external agent, for example, a microprocessor of a computer system.

Embodiments of the invention also encompass HDD 100 that includes the HGA 110, the disk 120 rotatably mounted on the spindle 124, the arm 132 attached to the HGA 110 including the slider 110 b including the head 110 a.

With reference now to FIG. 2, in accordance with an embodiment of the present invention, a plan view of a head-arm-assembly (HAA) including the HGA 110 is shown. FIG. 2 illustrates the functional arrangement of the HAA with respect to the HGA 110. The HAA includes the arm 132 and HGA 110 including the slider 110 b including the head 110 a. The HAA is attached at the arm 132 to the carriage 134. In the case of an HDD having multiple disks, or platters as disks are sometimes referred to in the art, the carriage 134 is called an “E-block,” or comb, because the carriage is arranged to carry a ganged array of arms that gives it the appearance of a comb. As shown in FIG. 2, the armature 136 of the VCM is attached to the carriage 134 and the voice coil 140 is attached to the armature 136. The AE 160 may be attached to the carriage 134 as shown. The carriage 134 is mounted on the pivot-shaft 148 with the interposed pivot-bearing assembly 152.

Adjusting the Water Vapor Permeability of a Desiccant Device Used within the Interior of Electronic Equipment

Embodiments of the invention advantageously provide for adjusting the water vapor permeability of a desiccant device to permit water to diffuse into the desiccant device at a faster diffusion rate than water is permitted to diffuse out of the desiccant device. As used herein, the term “desiccant device” refers to the physical enclosure that encloses or surrounds a desiccant material. A desiccant material is any material which is used to absorb water vapor within sensitive electronic equipment, such as, but not limited to, HDD 100.

Embodiments may be used to prevent the desiccant device from releasing humidity in the interior of HDD 100 at a rate too high to be desirable. In this way, the situation where the desiccant device releases a large amount of humidity within the interior of HDD 100 in a short period of time while the enclosure of HDD 100 does not have the capability to vent the water vapor quickly enough may be avoided. In fact, in some prior approaches, Applicant has observed that the permeation of water vapor through the enclosure of a HDD is about 50-100 times slower than the permeation of water vapor through the desiccant device used thereby, and in some cases it is thought this range may be even larger, such as 25-250 times slower than the permeation rate for water vapor for the desiccant device. Consequently, certain embodiments of the invention adjust the permeability rate of a desiccant device used within HDD 100 to match or roughly equate the permeability rate of the enclosure of HDD 100 in which the desiccant device resides. For example, in this specific example, the adjusted water vapor permeability rate of a desiccant device according to an embodiment would be adjusted to be 50-100 times slower than the original water vapor permeability rate of the desiccant device.

The water permeability of a HDD may be determined based upon analysis performed on the design of each model of HDD. In other words, in certain embodiments, it may not be necessary to test the water vapor permeability rate for each HDD that is manufactured, but instead, the water vapor permeability rate of a model of HDD may be determined anytime a change is made to the design of the HDD. For example, a specific HDD family may consist of several models with different capacities, but as long as the technology is the same for the enclosure, the ratio of drive to desiccant rates will be the same and does not need testing. If, however, as part of cost reduction a different cover gasket material is used, the water permeation rate of the HDD enclosure should be remeasured and the rate of permeation of the desiccant adjusted.

To illustrate a concrete example, assume that a particular HDD that resides in storage has an internal relative humidity (RH) of 60% at 30° C. After the HDD is powered on and begins operation, the interior of the HDD warms to a 65° C. running temperature. As a consequence, the absolute humidity (AH) will change considerably, for example from 11 g/m³ to 57 g/m³. While the HDD is running, the environment outside of the HDD may be 25° C. with a relative humidity (RH) of 70% and an absolute humidity (AH) of 11 g/m³. Thus, there will be a net flux of water out of the HDD to the external environment via diffusion.

However, as the water vapor concentration within the HDD decreases, the loss of water vapor in the interior of the HDD through diffusion will be offset by the influx of water vapor from the rapidly equilibrating desiccant, which is functioning as a reservoir and buffer for water vapor. By restricting the water vapor permeability rate of the desiccant device as accomplished by embodiments, the amount of water vapor expelled by the desiccant device at a given time should not be greater than the amount of water vapor lost to the outside of the HDD by diffusion.

An added advantage of lowering the water vapor permeability of the desiccant device will be to allow for less moisture absorption during manufacturing and/or to allow for more flexibility in terms of work-in-process (WIP) time for the HDD. For example, lowering the water vapor permeability of the desiccant device by 50-100 times to match the water vapor permeability of the HDD enclosure will lower moisture absorption by 50-100 times and allow for 50-100 times the amount of work-in-progress (WIP) time of the HDD.

Desiccant Devices May be Constructed in a Variety of Different Ways

In one embodiment, the material from which the desiccant device is made is selected to have a suitably low permeability to water. The purpose of doing so is to prevent a situation in which the water permeability rate of the desiccant device is an order of magnitude higher than that of the HDD enclosure. A desiccant device for use in a HDD is typically constructed of a formed or molded body enclosing the desiccant material. The desiccant device may, but need not, comprise a lid.

In prior approaches, the lid of the desiccant device, and possibly the entire body of the desiccant device, may be constructed of materials having a high water permeation rate. In contrast, according to embodiments of the invention, the body of the desiccant device is constructed out of a material to provide the desiccant device with a water permeation rate that matches or is substantially similar to the water permeation rate of the HDD enclosure.

There are several ways in which this may be accomplished. For example, all of the material used to construct the body of the desiccant device may have a suitable low water permeation rate that, coupled with available area of the desiccant device, matches as needed to the water vapor permeation rate of the HDD enclosure.

As another example involving a desiccant device having a lid, one material used to form the desiccant device may have a negligible water permutation rate, while another material used to form a different portion of the desiccant device (such as the lid) may have a permeation rate that, coupled with the area of that portion, matches appropriately with the water vapor permeation rate of the HDD enclosure.

As an additional example involving a desiccant device having a lid, the material of the desiccant body and the material comprising the lid may have a negligible water vapor permeation rate; however, the desiccant body may have a diffusion channel whose length and area create a restriction to water vapor that is suitably matched to the water vapor transmission rate of the HDD enclosure. Such an approach is depicted in FIG. 4, which is an illustration of a desiccant device having a diffusion channel according to an embodiment of the invention. Note that the diffusion channel depicted in FIG. 4 is not to scale.

In some cases, attention should be given to providing a means for equalizing the pressure between the interior and the exterior of the desiccant device. This equalizing means may be very small or even, in suitable cases, rely on diffusion of gas between the interior and the exterior of the desiccant device through the materials composing the desiccant device.

A desiccant device may be embodied in a variety of different forms. FIG. 3 is an illustration depicting several exemplary desiccant devices according to embodiments of the invention. As shown in FIG. 3, a desiccant device may be physically embodied as a variety of different structures, such as a pouch, a box, and a tub. The examples shown in FIG. 3 are not intended to depict all the forms in which a desiccant device may take, but are merely exemplary of several ways in which a desiccant device could be constructed.

As shown in FIG. 3, the contents of a pouch are entirely enclosed by a porous material, whereas the contents of a box or tub are partially non-porous. The non-porous surfaces of a desiccant device may be constructed using plastic material, while the porous surfaces of a desiccant device may be constructed using a microporous material that allows transmission of water vapor molecules therethrough. In an embodiment, the microporous material comprises at least one layer of expanded PolyTetraFluoroEthylense (ePTFE).

A desiccant device may be anywhere within the interior of HDD 100 in which the desiccant device is in communication with air space inside HDD 100. In an embodiment, the desiccant device may be affixed or positioned within the interior of HDD 100 such that the desiccant device is located in the flow of air when HDD 100 is operational.

Illustrative Examples of Adjusting the Water Permeability of Desiccant Devices

Several, non-limiting, illustrative examples of various embodiments will now be presented. In one embodiment, the desiccant device may be constructed using a polymer film having suitable water vapor permeability, i.e., the permeability of the desiccant device matches or is substantially similar to the water vapor permeability of the enclosure of HDD 100. An example of the form that the desiccant device may take is a tube, bag, pouch, box, or tub. The desiccant device may be comprised from a material such as a polyetheramide, polyurethane, polyester, or polycarbonate. The desiccant device in this example may be constructed to have a portion that is made using a non-woven fabric that is laminated to form a very thin diffusion barrier layer of less water permeable materials than the remainder of the desiccant device. The non-woven fabric may be laminated on either side of the diffusion barrier film.

According to another embodiment, a portion (the “body”) of the desiccant device is molded out of a material such as polypropylene so that the thickness and water permeability of the material making up the desiccant device body effectively prevents the transmission of water through the body of the desiccant device. The desiccant device may have a cover that is made of a material having a thickness and a water permeability rate to limit the transmission of water through the cover at a rate that matches or is substantially similar to the water permeability rate of the HDD enclosure. Such a material from which to construct the cover could be selected from polycarbonate, polyurethane, or similar materials. The material used to form the body of the desiccant device may be composed of a polymer film such that it is effectively water impermeable. Alternatively, the material used to form the body of the desiccant device may be an aluminized polyester or laminated foil used for moisture barrier bags. Other moisture barrier materials may be used to form the body of the desiccant device. Adjusting the water permeability rate of the desiccant device to match or be similar to the water permeability rate of the HDD enclosure may be achieved by forming a “window” or hole in the body of the desiccant device that is covered a different material. The type of material forming the desiccant device and the area of the window will affect the water permeability rate of the desiccant device. Examples of suitable materials for forming a desiccant device with a window are polyamides, polycarbonate, polyurethane, and a certain hard soft block copolymer material such as COPE. The window may be implemented using a window of a micro or nano porous material whose pore structure and density are selected to permit permeability matching with the HDD enclosure. Examples of such materials suitable for covering or forming the window are low stretched expanded PTFE and discrete pore filtering materials (such as EG Millipore™ and Nucleapore™).

According to another embodiment, the body of the desiccant device is formed out of a material such as polypropylene or polyester that, along with the selected thickness of the body, virtually prevents the transmission of water through the body of the desiccant device. In this embodiment, the desiccant device may comprise a lid that is comprised of similar materials. The lid may be constructed to have a channel (a “diffusion limiting channel”) that limits diffusion by the ratio of cross sectional area to length. Exemplary dimensions of such a diffusion limiting channel are 14 mm long by 0.3 mm deep by 0.7 mm wide, giving an A/L (area/length) of approximately 0.015 cm. FIG. 4 depicts (not to scale) a desiccant device having a diffusion limiting channel according to an embodiment. These dimensions should be designed to provide the needed resistance to render the water permeability rate of the desiccant device to match or be substantially similar to the water permeability rate of HDD enclosure.

There are several ways to construct portions of a desiccant device of an embodiment to prevent or substantially prevent the transmission of water therethrough. For example, metail foil laminates and evaporated metal may be used to construct portions of a desiccant device, and such materials substantially prevent the transmission of water therethrough. Other examples of suitable materials include sputtered or ChemVapor Deposited materials such as silica and a laminate with a high resistance plastic (such as 3M ACLAR film which is a trade name for fluorochloro ethylene polymers).

The body of the desiccant device may be formed of a tube or bag formed from one laminate of a film having a low moisture transmission rate and another film having a high moisture transmission rate. The latter may be a nonwoven fabric or a micro porous material such as expanded PTFE. A film layer may be perforated by needle punch, laser ablation, spark discharge, or any other method to produce a plurality of holes of a very small and generally controlled size. The area of a hole and the density of the film create the restriction to the transmission of water therethrough.

A laminate from which a desiccant device may be made from may comprise multiple layers. For example, if the laminate comprises three layers, then the middle layer may be composed of dense non-woven fabric (such as calendared airlaid polyester) and the outer layers may be comprised of layers perforated by needle punch, laser ablation, spark discharge, or any other method to produce a plurality of holes. The dense laminate forms a restrictive layer between the two layers of small perforations. The process can be repeated to enhance resistance to the transmission of water vapor. Another manner of making filter media is etching a plurality of small holes following exposure to radiation or by other means to produce very fine holes in a layer.

In an embodiment, the desiccant device body may be molded using a material having a low water vapor transmission rate and a cover made from a similar material. The cover may be perforated by one or more micro capillaries of glass, plastic, or metal. The cross sectional area of the micro capillaries along with the length and number of the capillary tubes regulate the water permeation rate.

In another embodiment wherein the desiccant device comprises a body and a cover (or lid), the body and cover of the desiccant device are attached by means of a formed rubber gasket, such that high doses of radiation used to cure the gasket will produce a rough surface. The rough surface of the gasket creates a large plurality of very small channels. By adjusting the gasket material, does of radiation, fillers, etc., the water permeation rate of the gasket may be adjusted to result in the desiccant device having a matching or similar water permeability rate as the enclosure of the HDD.

It is noted that the particular means by which water permeation is limited to, and out of, the desiccant device may vary across embodiments, as illustrated in the discussion above.

Constructing a Desiccant Device Using a Differentially Permeable Membrane

In certain embodiments, neither the packaging in which HDDs are stored and shipped nor the gaskets that seal the HDD against contamination may be impermeable to water. The low cost formed in place gaskets (FIPG) and the low cost dispensed in place gaskets (DIPG) used within HDDs are generally moderately permeable to water. When a HDD is manufactured, all components will be at or nearly at the equilibrium humidity of the manufacturing clean room. In some cases, water from the cleaning process may be incompletely removed from the HDD. A desiccant device is often used inside a HDD for purposes of lowering the humidity after mechanical assembly of the HDD and before the HDD completes processes such as servowriting and testing. Desiccant devices may also serve as the primary protection against exposure of the HDD to moisture during shipping or storage.

One of the typical properties of a prior art desiccant device for use within a HDD is that the desiccant device allows for the free transmission of water vapor therethrough. Thus, even if the transmission is restricted, the rate of absorption is the same as the rate of desorption. Thus, if a desiccant will rapidly remove water from the interior of the HDD after the interior of the HDD reaches equilibrium with the environment outside of the HDD, once the temperature in the interior of the HDD rises, then water vapor in the desiccant will be emitted to maintain a relative humidity within the interior of the HDD. Constant relative humidity (RH) with rising temperature causes a rise in absolute humidity or water vapor concentration. An effective equilibrium will be maintained resulting in days or even weeks of high water vapor concentration after the HDD is placed in service.

FIGS. 5A-5B are graphs that illustrate the interaction of relative humidity, absolute humidity ([H₂O]), and temperature. As shown in FIGS. 5A-5B, although relative humidity will remain constant as the temperature increases, water vapor concentration will increase, which poses a risk to the HDD. This is so because the amount of water in a vapor state within a HDD is small compared to the amount of water within materials inside the HDD, such as plastic and a desiccant. As a result, when the temperature rises, water within such inner materials of the HDD is released.

FIG. 6 is a graph that illustrates a change in the fly height measured as a function of temperature and relative humidity inside of a HDD. As shown in FIG. 6, the fly height of a HDD decreases with an increase in relative humidity inside the HDD, thereby threatening the normal operation of the HDD.

The Brown patent application, incorporated by reference herein, teaches approaches for controlling humidity levels inside of a HDD by balancing the rate at which moisture will diffuse out of a desiccant device with that of the HDD enclosure. Embodiments of the present invention provide approaches for absorbing moisture from the interior of the HDD into the desiccant device at a rate faster than moisture will be emitted from the desiccant device into the interior of the HDD. Embodiments of the invention may be, but need not be, used in conjunction with approaches taught by the Brown patent application.

One way in which desiccant devices may be constructed to absorb moisture from the interior of the HDD into the desiccant device at a rate faster than moisture can diffuse from the desiccant device into the interior of the HDD is to form the desiccant device using laminate 700 of FIG. 7A. FIG. 7A depicts laminate 700 from which at least a portion of a desiccant device according to an embodiment of the invention may be constructed. Laminate material 700 may be constructed using exterior facing material 702 and interior facing material 704. Note that while only two layers in laminate 700 are depicted in FIG. 7A, laminate 700 may be composed of three of more layers in other embodiments of the invention. If the relative humidity outside the desiccant device is larger than the relative humidity inside the desiccant device, then the rate of migration of water across the multipart membrane is significantly larger than that when the relative humidity inside the desiccant device is larger than the relative humidity outside the desiccant device.

Laminate 700 may be composed of different materials that are selected based on their water solubility and diffusion rates. The particular materials selected as exterior facing material 702 and interior facing material 704 will affect the overall diffusion rate of water from both sides of laminate 700. While a variety of materials may be used for exterior facing material 702 and interior facing material 704, exterior facing material 702 should have a higher water solubility than interior facing material 704. In an embodiment, exterior facing material 702 should be plasticized by water vapor whereas interior facing material 704 should not be plasticized by water vapor. By selecting a material for external facing material 702 that is plasticized by water vapor, the rate of moisture transmission from the outside of the desiccant device to the inside of the desiccant device will increase as the water content in external facing material 702 increases. In an embodiment, interior facing material 704 is not highly resistant to water vapor to avoid an overly slow diffusion rate for laminate 700.

To illustrate a concrete example, exterior facing material 702 may be a water plasticized and highly water absorbing material with a high ratio of functionality to hydrocarbon backbone and interior facing material 704 may have a low water solubility and be devoid of polar functionality. A specific embodiment employs polyamide 6 as exterior facing material 702 while employing a high flexible medium density polyethylene as interior facing material 704. Other embodiments use PA-66 or PA-4 as exterior facing material 702 and ethylene-tetrafluooethylene copolymer (ETFE) as interior facing material 704. A commercial example of PA-66 or PA-4 is Nylon™ and a commercial example of a copolymer of ethylene and tetra fluoroethylene is Tefzel™ by DuPont. Using a block polymer (e.g., COPE or PEBA) as exterior facing material 702 inhibits the differential effect even though a block polymer has a very high water diffusion rate, as does using a rigid material for interior facing material 704 with a lower water solubility for (e.g., polypropylene or Fluoroethylene propylene (FEP)), as shown in FIGS. 8A-8B.

FIG. 7B is an illustration of desiccant device 710 according to an embodiment of the invention. At least portions of desiccant device 710 may be constructed using the laminate depicted in FIG. 7A. Desiccant device 710 may be used inside a server class HDD or HDD 100 for example. As shown in FIG. 7B, desiccant device 710 will absorb moisture significantly faster than desiccant device 710 will release moisture. As explained above, the outgoing diffusion rate of desiccant device 710 may be designed to be equal to or approximate to the water vapor diffusion rate from the interior of the HDD in which desiccant device 710 resides to the exterior environment of the HDD in which desiccant device 710 resides.

FIGS. 8A-8B are graphs illustrating data concerning water vapor diffusion through laminate 700 according to an embodiment of the invention. The data depicted in FIGS. 8A-8B depict a test wherein laminate 700 is placed across an opening (or “window”) of a desiccant device and the humidity is measured inside the desiccant device. As shown in FIGS. 8A-8B, there is small lag (the lag is larger when the moisture is permeating from interior facing material 704) while the water vapor is penetrating laminate 700 and establishing a kinetic steady state. As shown in FIGS. 8A-8B, once the water vapor penetrating laminate 700 from the exterior facing side to the interior facing side establishes a kinetic steady state, the movement of the water vapor is at a relatively steady rate.

In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. Thus, the sole and exclusive indicator of what is the invention, and is intended by the applicants to be the invention, is the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. Any definitions expressly set forth herein for terms contained in such claims shall govern the meaning of such terms as used in the claims. Hence, no limitation, element, property, feature, advantage or attribute that is not expressly recited in a claim should limit the scope of such claim in any way. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. 

What is claimed is:
 1. A desiccant device for use in an enclosure protecting sensitive electronic equipment, comprising: the desiccant device enclosing a desiccant material, wherein the desiccant device permits water to diffuse from the interior of the hard-disk drive into the desiccant device at a faster diffusion rate than water is permitted to diffuse out of the desiccant device into the interior of the hard-disk drive, wherein the desiccant device comprises no channels between the exterior of the desiccant device and the interior of the desiccant device, wherein the desiccant device is constructed using a laminate material, wherein the laminate material comprises (a) an exterior facing material which is exposed to the interior of the hard-disk drive and is not exposed to the interior of the desiccant device, and (b) an interior facing material which is exposed to the interior of the desiccant device and is not exposed to the interior of the hard-disk drive, and wherein the exterior facing material permits water to diffuse faster therethrough than the interior facing material, wherein the laminate material comprises an inner layer that is disposed between the exterior facing material and the interior facing material, and wherein the inner layer is composed of dense non-woven fabric.
 2. The desiccant device of claim 1, wherein the interior facing material is a hydrophobic polymer, and wherein the exterior facing material is a hydrophilic polymer.
 3. The desiccant device of claim 1, wherein the exterior facing material is a water plasticized material having a high ratio of functionality to hydrocarbon backbone.
 4. The desiccant device of claim 1, wherein the interior facing material is a material with a low water solubility and is devoid of polar functionality.
 5. The desiccant device of claim 1, wherein the interior facing material is a medium density polyethylene.
 6. The desiccant device of claim 1, wherein the desiccant device is also constructed using a water impermeable material, and wherein the laminate material comprises a minority portion of the body of the desiccant device.
 7. The desiccant device of claim 1, wherein the water diffusion rate of the interior facing material matches or is substantially similar to the water vapor permeability rate of the enclosure.
 8. The desiccant device of claim 1, wherein the interior facing material is aluminized polyester or laminated foil, and wherein exterior facing material is water vapor permeable.
 9. The desiccant device of claim 8, wherein the exterior facing material is low-stretched expanded PolyTetraFluoroEthylense (PTFE) or a discrete pore filtering material.
 10. The desiccant device of claim 1, wherein the inner layer is calendared airlaid polyester.
 11. A hard-disk drive, comprising: an enclosure which encloses an interior of the hard-disk drive; a magnetic-recording head; a magnetic-recording disk rotatably mounted on a spindle; and a desiccant device enclosing a desiccant material, wherein the desiccant device permits water to diffuse from the interior of the hard-disk drive into the desiccant device at a faster diffusion rate than water is permitted to diffuse out of the desiccant device into the interior of the hard-disk drive, and wherein the desiccant device comprises no channels between an exterior of the desiccant device and an interior of the desiccant device, wherein the desiccant device is constructed using a laminate material, wherein the laminate material comprises (a) an exterior facing material which is exposed to the interior of the hard-disk drive and is not exposed to the interior of the desiccant device, and (b) an interior facing material which is exposed to the interior of the desiccant device and is not exposed to the interior of the hard-disk drive, and wherein the exterior facing material permits water to diffuse faster therethrough than the interior facing material, wherein the laminate material comprises an inner layer that is disposed between the exterior facing material and the interior facing material, and wherein the inner layer is composed of dense non-woven fabric.
 12. The hard-disk drive of claim 11, wherein the interior facing material is a hydrophobic polymer, and wherein the exterior facing material is a hydrophilic polymer.
 13. The hard-disk drive of claim 11, wherein the exterior facing material is a water plasticized material having a high ratio of functionality to hydrocarbon backbone.
 14. The hard-disk drive of claim 11, wherein the interior facing material is a material with a low water solubility and is devoid of polar functionality.
 15. The hard-disk drive of claim 11, wherein the interior facing material is a medium density polyethylene.
 16. The hard-disk drive of claim 11, wherein the desiccant device is also constructed using a water impermeable material, and wherein the laminate material comprises a minority portion of the body of the desiccant device.
 17. The hard-disk drive of claim 11, wherein the water diffusion rate of the interior facing material matches or is substantially similar to the water vapor permeability rate of the enclosure.
 18. The hard-disk drive of claim 11, wherein the interior facing material is aluminized polyester or laminated foil, and wherein exterior facing material is water vapor permeable.
 19. The hard-disk drive of claim 18, wherein the exterior facing material is low-stretched expanded PolyTetraFluoroEthylense (PTFE) or a discrete pore filtering material.
 20. The hard-disk drive of claim 11, wherein the inner layer is calendared airlaid polyester. 